Generally, organic light emission means that electric energy is converted into light energy by using an organic substance. An organic light emitting device (OLED) using the organic light emission typically includes an anode, a cathode, and an organic material layer that is interposed between the anode and the cathode. The organic material layer is to have a multilayered structure made of different substances in order to improve efficiency and stability of the organic light emitting device. For example, the organic material layer may be formed of a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, etc. If voltage is applied between two electrodes in the organic light emitting device having the above-mentioned structure, a hole is injected into the organic material layer at an anode and an electron is injected into the organic material layer at a cathode. When the hole meets the electron, an exciton is generated, and light is generated when the exciton is converted into a bottom state. It is known that the organic light emitting device has properties such as self-light emission, high luminance, high efficiency, a low driving voltage, a wide viewing angle, high contrast, and a high-speed response.
Various types of organic light emitting devices are known in the related art and they may be used for different applications. Examples of an organic light emitting device include a top light emitting OLED, a bottom light emitting OLED, and a dual-sided light emitting OLED.
If bottom light emitting OLEDs are used in active matrix displays, thin film transistors (TFT) are provided in front of the source of light emission, thereby reducing the ratio of the effective display area (aperture ratio). This problem is significant in the case when sophisticated displays having many TFTs are manufactured. With respect to a bottom light emitting OLED having an aperture ratio of less than 40%, an estimated aperture ratio of a WXGA type display that includes a TFT for 14″ grade is less than 20%. Such small aperture ratio negatively affects the driving power consumption and lifetime of the OLED.
The above-mentioned problem can be prevented by using a top light emitting OLED. In a top light emitting OLED, an electrode that is not in contact with a lower substrate, that is, an upper electrode, is substantially transparent in a visible ray region. A transparent electrode that is used to form the upper electrode of the top light emitting OLED is formed of a conductive oxide, such as IZO (indium zinc oxide) or ITO (indium tin oxide), or a thin film made of metal, such as MgAg, Al, Ag, Ca, and CaAg. However, an electrode that is in contact with the substrate is typically made of metal. Similar to the top light emitting OLED, the dual-sided light emitting OLED includes a transparent upper electrode.
FIG. 1 illustrates a lower portion of an inverted organic light emitting device. As shown in FIG. 1, when forming a top light emitting OLED, after a metal cathode is deposited on a substrate, a undesirable native oxide layer is formed on a surface of a cathode. In detail, when the metal cathode is patterned by using photolithography and etching processes during the fabrication of an organic light emitting device, the metal cathode is exposed to moisture and oxygen, causing a native oxide layer to be formed on the metal electrode.
The native oxide layer reduces properties of the cathode, that is, electron injection properties, thus reducing efficiency and luminance of the organic light emitting device.
One process to prevent the native oxide layer from being formed on the cathode is to form an organic material layer on the deposited cathode in situ. During the process, since the cathode is not exposed to air, the oxide layer is not formed on the surface of the cathode. However, it is costly and difficult to perform both a process of forming a TFT substrate including a cathode and a process of forming an organic material layer and other layers in a vacuum in situ.
In addition, a raw material supplier sometimes supplies a TFT substrate, on a surface of which a cathode is layered, while the substrate is exposed to air before an organic substance is deposited. In this case, there is a problem in that the native oxide layer is formed on the cathode. Therefore, in the related art, a TFT substrate that includes an anode made of a substance such as ITO is frequently used. However, in this case, since a TFT process line of a conventional LCD process of fabricating an n-type TFT where electrons are injected cannot be used, and the TFT substrate including an oxide electrode such as ITO is used to manufacture a bottom light emitting OLED, it is not suitable to manufacture a top light emitting OLED.
Therefore, there is a need to develop an organic light emitting device having improved electron injection properties, even though the native oxide layer is provided on the metal cathode, and a method of fabricating the same.